Soil Water Up, Nitrogen Down

Thunder is booming outside as I write this on the morning of April 16. It is 8:30 and we’ve already had 3/4 of an inch of rain today.

In the past 10 days, we’ve had 3.5 inches of rain at our research farm near Columbia, and 5.5 inches in the past 40 days.

A soil sample taken two weeks ago at this farm showed 30% soil moisture in the top 1.5 feet and 25% moisture from 1.5 to 3.5 feet. Not bad, considering how dry it was last fall.

A followup sample taken a week ago showed 25% moisture in the top foot and 30% below that–water had shifted downward. A lot of progress has been made to replenish the water in the subsoil, which was left unusually dry following last year’s drought.

While not everyone has had as much precipitation as we have in Columbia, most of Missouri has had more than 6 inches since the first of the year. (The 6 counties in the northwest corner are the exceptions.) Very little of this water has evaporated back to the air, as it has been a cool late winter and spring.

The story with nitrogen is, if anything, the opposite. I have been telling people that if they’re planting corn after corn (or planted wheat after corn), and their corn yields last year were low, they can probably take a credit for some of the leftover N and cut back rates. I’m now worried about this suggestion--two recent deep soil samples suggest that at this point a credit for last year’s N should be low.

One sample was taken by Bud Motsinger in Caldwell County. His results came back suggesting that a 30 to 40 lb N/acre credit would be appropriate. This was for a field that received 180 lb N/acre but didn’t yield much. There should have been a lot more N left at harvest than he saw in his recent sample, suggesting that some has been lost.

We took a sample a week ago at our research farm near Columbia in corn that had received 180 lb N/acre and yielded 80 bushels. Figuring 1 lb N per bushel ends up in grain and stover, that would leave 100 lb N/acre unused. What we measured in our soil sample to a depth of 3 feet was similar to what Bud saw, suggesting about a 30 lb N credit.

Where did the N go? It would have mostly been in the nitrate form at harvest, a form Missouri total precipitation, January 1 to April 15 that moves freely with water. We found almost zero nitrate in the top foot, but about 20 lb N/acre as nitrate in the 2nd foot and another 20 in the 3rd foot. In short: the nitrate has moved down, and it seems likely that a good bit has moved below 3 feet deep. The chances that corn roots can go down and get this N are not great. It doesn’t seem to me like it’s been wet enough to leach out most of the nitrate left in the soil at harvest, especially in a slowly-draining soil like this one, but the numbers suggest that that’s what has happened.

So if you have taken a credit for last year’s leftover N, or were planning to–PROCEED WITH CAUTION. These results are only from two fields, and other fields are sure to be different, but they suggest that the risk that comes with taking a nitrogen credit is higher than I would have guessed.

For wheat, time is rapidly running out to correct any potential problems. I would suggest quickly scouting any fields where a credit was taken, and getting an additional 30 lb N/acre applied soon to fields where any sign of N deficiency is seen.

For corn fields where a credit was taken and N was applied at a lower-than-normal rate, I suggest the same thing–watch carefully and run some more nitrogen over the field at the first sign of deficiency.

For corn fields where N has not yet been applied, I would suggest a small credit, no credit, or a deep (2 feet) soil sample for nitrate and ammonium to see if a larger credit is justified. Taking the soil sample from one or two fields would give you a rough guide to the current status of soil N in your area (see MU Extension guide G9177 for details).

It’s great to have water back into our subsoils to contribute to the water needs of this year’s crops. The downside is that some of the N that was applied last year but not used may no longer be available for use by this year’s crop.

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